Selective wave transmission



Nov. 19, 194%, D. K. ORAM SELECTIVE WAVE TRANSMISSION Filed June 28, 1-939 2 Sheets-Sheet 2 I 1 INVENTOR flu Laid am/ 'BY Patented Nov. 19, 1940 Ul'lED S'l'AES NT 'Ql' 'l E' SELECTIVE WAVE TRANSMISSION New York Application June 28, 1939, Serial No. 281,612

8 Claims.

This invention pertains to electrical apparatus and circuits of the type known as filters and more especially to such apparatus and circuits of the type referred to as band pass filters.

One object of my invention is to provide a band pass filter circuit capable of affording passage therethrough to a band of frequencies, the width of which band is susceptible of variation, as by manual control.

Another object of my invention is to provide a filter circuit which will, when used in connection with a radio receiver, allow the degree of selectivity of such receiver to be easily varied over a predetermined range, at the will of the operator of such receiver.

Another purpose of this invention is to provide in a band pass filter circuit, means for varying by a single controlling adjustment, the Width of the band of frequencies effectively passed by such filter circuit.

r A further purpose of this invention is to provide a tuned amplifier circuit Whose degree of selectivity can readily and quickly be varied, without the resonant frequency'of such amplifier circuit being substantially affected by such variation of its selectivity.

Yet another object of my invention is to provide a combination coupling and filter circuit suited for use in the intermediate frequency amplifier portion of a superheterodyne type of radio receiver and having a single control element for varying the response characteristics of such a receiver with regard to its selectivity without at the sametime materially altering the peak resonant frequency of such amplifier and receiver.

A still further purpose of this invention is to provide, in a circuit of the character just described, simple and readily operated control means for altering the capacity in one portion of the circuit, while at the same time making a corresponding alteration, but in the other sense, of the capacity in another portion of the circuit, so that the total capacity influencing one of the frequency discriminating elements of the circuit will be kept substantially constant. specifically stated, a neutralizing capacity may be changed in value while the total effective capacity influencing a resonant circuit to which the neutralizing capacity is connected, Will remain constant.

Yet another object of my invention is to reduce the noise to signal ratio in a radio receiver, by reducing the band pass width of a filter circuit More incorporated in such receiver to such a degree as may be found necessary, and to make such reduction quickly and to a predetermined degree.

Another purpose is greatly to attenuate or virtually to eliminate unwanted signals received at a frequency closely adjacent the frequency at which desired intelligence bearing signals are being received and to narrow the band pass width of a filter circuit until such desired signals can be received through such extraneous and unwanted signals. 1

Another purpose of my invention is to combine a crystal resonator having a fixed value of Q with a resonant circuit having a variable Q, so as to provide a combination filter circuit Whose 15 overall transmission characteristics may be so altered as to allow the effective transmission of a band of frequencies, the width of this band i being susceptible of variation within predetermined limits by. such alteration of transmission characteristics. 5 I

Yet another object of my invention is to; provide a combination filter circuit of the character just described which shall enable the operator of a radio receiver incorporating such circuit 2 therein, to change the response characteristic of such receiver so as to obtain degrees of selectivity lying between the extremely sharp selectivity afiorded by a quartz crystal resonator and the relatively broad response of a' coil-condenser circuit.

Another purpose of my invention is to providea radio receiver wherein a single variable resistance is employed to control the overall selectivity of the receiver without substantially altering the resonant tuning thereof.

A still further object of this invention is to provide a radio receiver having a frequency band pass characteristic .of' easily variable efiective width, but having at all times a substantially 40 constant symmetry of transmission characteristics.

Yet another purpose of my invention is to provide a radio receiver with a single switch which shall constitute the only moving member 4 necessary for varying the selectivity of such receiver, such switch furnishing'a simple and noncritical means of adjustment.

Another purpose ofmy invention is, in a com- 50 bination filter circuit, to provide a single variable or tapped resistor, the alteration of the efiective resistance of which will bring about a change of selectivity of a receiver incorporating such circuit,

extendingfrom the extremely great selectivity of 5 5;

a receiver using a simple crystal resonator to the relatively slight selectivity of a coil-condenser type of receiver, and also allowing the receiver selectivity to take on intermediate values lying at a plurality of desired points between these two limits.

A yet further purpose of this invention is to provide a tuned filter circuit of thevariable band pass type which can be used in conjunction with any apparatus employing or utilizing a plurality of frequencies, such as harmonic generators, wave analyzers, radio transmitters and the like, for the purpose of passing certain frequencies and suppressing certain other frequencies.

Other uses and advantages of this invention will be apparent to those skilled in the art from the following specification and drawings.

Fig. 1 is a schematic diagram showing a modified resonant circuit of the parallel coil-condenser type, for the purposes of illustrating certain principles employed in my invention.

Fig. 1A is a schematic diagram showing the circuit of Fig. 1 combined with a crystal resonator so as to form a filter network constituting the load circuit of a transformer.

Fig. 2 is a graph showing the reduction in the resonant impedance of the circuit of Fig. l as additional resistance is introduced therein.

Fig. 3 is a diagrammatic representation of a circuit showing one embodiment of my invention, wherein the circuit of Fig. 1A is combined with other elements contributing to its practical operation.

Fig. 4 is a schematic showing of a modified form of the circuit of Fig. 3 connected between two electron tubes so as toform a coupling circuit therebetween.

Fig. 5 shows a family of selectivity curves illustrating the functioning of a typical filter and coupling circuit embodying my invention.

Fig. 6 is a schematic diagram which illustrates one form of a complete filter and coupling circuit embodying my invention, as applied to the beat frequency amplifier of a superheterodyne type of receiver.

In most radio receiving apparatus in general use, selection of the desired signals is accomplished by the use of resonant circuits comprising inductances and capacities connected together so as to form either series or parallel tuned circuits. The degree of selectivity afforded by such a tuned circuit is determined by the ratio of its reactance to its resistance, customarily expressed in the art by the symbol Q. At the frequencies commonly employed as beat frequencies in the superheterodyne receiver, e. g. 455 kc., it .has been found impractical because of losses in inductances and for other reasons, to provide circuits having a. Q greatly exceeding 200, and in general the Q of such circuits falls considerably below that figure.

In some special applications the signal selecting device used is one wherein a resonating piezoelectric crystal is employed. Circuits using such crystals are well known in the art and the choice of a material for the crystal, its preparation and mounting so that it will resonate at a certain desired frequency are so well known as to need no detailed description. Such a crystal resonator may be considered as the equivalent of a coilcondenser circuit presenting an extremely high. Q, of the order of 10,000 or more.

Due to the Widely different values of Q for thetwo types of circuits just described, the selectivity of a crystal resonator may be of the order of times as great as that of a coil condenser type circuit. In actual practice, the Q of the quartz crystal resonator alone has been too great to allow its use with receivers designed to pass radio signals modulated at voice frequencies, i. e., it

.has been found commercially unsuccessful to employ such simple crystal filters in radio broadcast receivers where a response of high quality is desired. Therefore its use has generally been confined to receivers designed for key-modulated ra dio signals, i, e., radio telegraph receivers.

For certain types of service required of radio receiving apparatus, neither of the above described degrees of selectivity is completely satisfactory. For the reception of voice-frequency modulated signals whose carriers are separated from each other by a sufficient number of kilocycles, as in the broadcast band, coil-condenser selector circuits provide the necessary selectivity without undue attenuation of the sidebands so necessary to the faithful reproduction of speech and music. However, in other frequency bands where the separation between modulated carriers is not fixed and may be as little as one kilocycle or even less, such circuits do not afford sufficient selectivity to insure reception of a desired signal through closely adjacent interfering signals. On the other hand the extreme selectivity of the quartz crystal resonator results in such severe attenuation of the sidebands as to almost completely'obliterate the audio-frequency component of the received signal.

The so-called communications type receiver is intended for reception of both voice-modulated signals and key-modulated or telegraph signals over a wide range of frequencies, and in bands Where the separation between adjacent carriers varies between wide limits. As is well known in the art, the selectivity and fidelity of reproduction of a radio receiver are inversely related to each other. For this reason it follows that for a given frequency separation between a desired signal and an adjacent interfering signal there is a certain degree of selectivity that will result in the best compromise between sideband attenuation of the desired signal and intolerable interference from the undesired signal. It is thus to be'seen that such a receiver should possess several greatly different degrees of selectivity in order to afford the best reception possible regardless of the frequency separation between a desired signal and adjacentinterfering signals.

I have discovered that by a suitable combination of a resonant crystal with a coil-condenser circuit, I may obtain a filter circuit whose oven all transmission characteristics will lie between those possessed by the simple resonant crystal with its extremely high Q, and those possessed by the usual coil-condenser circuit with its relatively low Q.

Furthermore I have found that by an easily effected and relatively simple alteration in the characteristics of the coil-condenser circuit of such a combination I can readily and quickly vary the degree of selectivity of the combination between wide limits, the maximum selectivity being determined by the Q of the crystal resonator alone and the minimum selectivity being dctermined by the electrical constants of the coilcondenser circuit. Thus I am enabled to provide a single radio receiver which may be employed both for telegraph and for speech reception and whose several degrees of selectivity may be used so as to afford respectively. excellent quality reception of speech, reduced quality reception of speech with increased rejection of interfering signals, and extremely selective reception of telegraph signals. Furthermore the change from one degree of selectivity to another in a receiver constructed according tomy invention may be obtained by a single, easily operated manual control, the operation of which will be substantially without secondary effects, such as detuning the receiver or the introduction thereinto of asymmetrical transmission characteristics.

Referring now to Fig. 1 I have here illustrated a parallel resonant circuit of the coil-condenser type. The coil 20 and condenser 2| are connected in parallel with one another. This circuit differs somewhat from the usual circuit of this type in that I have also incorporated a discrete resistance 22 in series between condenser 2| and coil 20, this resistance being connected tothe coil by conductor 23. Lead 24 serves to directly connect condenser 2| and coil 20 and lead 25 serves to connect condenser 2| with resistance 22. Such a circuit exhibits relatively high impedance to alternating currents whose frequencies correspond more or less closely with its resonant frequency. The impedance across points 26 and 21 is indicated by the symbol Z.

The characteristics of a parallel resonant circuit of the type of Fig. 1 are well known in. the

art, as also is the fact that the magnitude of its impedance will vary inversely with the amount of resistance present therein. Ordinarily the Q of this type of circuit is such that additional resistance over that inherently and unavoidably possessed by the coil and condenser is not introduced. However, I have found that the introduction of such additional resistance and more specifically its introduction to a varying degree, may be advantageously utilized in carrying out the purposes of my invention.

Referring now to Fig. 1A, there is here shown a circuit including coil 20, condenser 2| and resistance 22, of the type just described, and in series with this circuit there is indicated a piezoelectric resonator 28 which may be of any type such as the usual quartz crystal type. The crystal resonator and the coil condenser circuit are in series with one another, being joined at the point 26 and the combination is connected by conductors 29 and 30 to secondary winding 3! of a transformer 32, the input of the primary 33 of which is derived through leads 34, 34. The output of this combination filter is obtained from the points 26 and 21 by means of leads 35, 35.

Referring now additionally to Fig. 2, there is here illustrated a curve showing the reduction of the impedance of the circuit of Fig. 1 as additional resistance is introduced therein. The magnitude of the impedance is shown vertically by scale 36, while the amount of added resistance is indicated along the horizontal axis of the figure by scale 31. The addition of this resistance may be conveniently accomplished by the variable resistor R (22) of Fig. 1A. The portion 38 of the curve shows the rapid drop of impedance at first, while portion 39 shows the slower subsequent drop of impedance. While the additional resistance is most conveniently added by means of a series resistor, it is well known in the art that the effective series resist ance of a tuned circuit may likewise be increased by placing resistance in shunt thereto and there-- fore while the following discussion assumes a series resistance, it is to be understood that a shunt resistance having equivalent effect in reducing the impedance of the circuit may be optionally employed.

At this point a discussion of the underlying principles upon which the operation of the circuit of Fig. 1A is based, is thought to be ad- Visable. For the purposes of this discussion it will be assumed that the natural period or resonant frequency of both the quartz crystal resonator 28 and the parallel tuned circuit LCR is precisely 455,000 C. P. S., and all references to natural period or resonant frequency appearing in this specification shall be taken tomean that exact frequency, solely for purposes of illustrative example. Furthermore, the by-passing effect of the electrostatic capacity possessed by the crystal and its holder will be ignored until a later point of this specification, as it is Well known in the art'that such effect may be counterbalanced or neutralized by appropriate apparatus and devices.

When resistance 22 is set at its minimum value,

preferably zero, the parallel tuned circuit LC of.

Fig. 1A will still possess a certain resistance determined by the inherent resistance possessed by the coil 20 and capacity 2|. As a typical example we may assume such a combination to have an efiective resistance of 24 ohms and reactances of 3200 ohms, thus possessing a Q of 133. The parallel impedance (at resonant frequency) will be equal to I or QX or 425,000 ohms. On the other hand, the quartz crystal resonator may be assumed to have an effective resistance of 4000 ohms, reactances of 60,000,000 ohms, and Q of 15,000. Its series impedance (at resonant frequency) will thus be 4000 ohms..

For the purpose of explaining the operation of my invention, I find it desirable to'regard the crystal resonator 28 and the parallel tuned circuit LCR. in series with it from the Viewpoint hereinafter described. It is to be understood that I do not in any way confine the operation of my invention to this particular theory of operation or to any other theory, since the operation of the invention is factually discernible by Wellknown means of measurement and observation.

For the purposes of the theoretical explanation now to be given, it is to be assumed that the voltage delivered by conductors 29 and 30 (Fig. 1A) to points 28 and 21 respectively of a combination filter, remains substantially constant over the relatively narrow band width being considered. One method of securing such a substantial constant voltage is to make secondary winding 3| of transformer 32, of a relatively low impedance, but any other means familiar to the art may be employed to secure this same constancy of voltage. It is to be understood that such constancy of input voltage to points 28. and 21 is not necessary for the proper operation of my invention; its assumption merely eliminates one variable, thus simplifying this particular explanation of its operation. It is now possible to consider the crystal resonator 28 and the parallel tuned circuit LCR, connected in series between points 27 and 28', as two discrete portions of a voltage divider. Under this guise point 20 becomes the equivalent of the intermediate tap in such a voltage divider and consequently since the output circuit'is connected to points 26 and 21, the voltage, E, developed across those two points will bear some definite relation to the voltage across the input terminals of such voltage divider, i. e., across points 21 and 28. I

The difference between my combination filter looked on as a voltage divider, and the ordinary type of voltage divider, resides in the unusual electrical characteristics of its two series connected sections. The characteristics of each section of my Voltage divider are subject to great changes in accordance with. the frequency of alternating current impressed thereupon. Since the voltage across the two sections in series is considered to be substantially constant, it necessarily will follow that the amplitude of the current flowing will depend directly on the total impedance presented by the entire divider, and the voltage E will be equal to the product of this current and the impedance presented by that section of the divider between points 26 and 21. It will furnish a convenient method of explaining the selectivity characteristic of this divider, to consider the change of impedance in each of its sections, for small changes in impressed frequency, on either side of the resonant frequency.

Accordingly there is given below a table showing the changes in impedance. Column I shows the degree of departure from the resonant frequency of 455,000 cycles. In column II is given the corresponding impedance of the quartz crystal resonator. In columns III and IV are shown the corresponding impedance values of the parallel tuned circuit LCR with no added resistance, and with 2000 ohms added, respectively. The numerical values, while rounded 011, are sufiiciently close for purposes of illustration.

Although the constants of the two sections differ so enormously from each other that even small changes in the impressed frequency produce great dilferences in the relation between their respective impedances, upon consulting the above table it will be noted that when no additional resistance is introduced in the parallel tuned circuit LCR, the total impedance of the two sections in series changes but slowly as the impressed frequency departs from resonance. This is by reason of the fact that the steeply rising impedance of the crystal resonator consists almost entirely of reactance, even for the comparatively slight frequency deviation of :50 cycles, whereas the resistive component of the impedance of the parallel tuned circuit LCR predominates for frequency departures up to :1706 cycles. At these two frequencies, namely 453,294 cycles and 456,706 cycles, the reactive and resistive components are equal to each other and to 213,333 ohms; thus at these two frequencies the impedance of the circuit LCR is equal to 301,700

ohms. Furthermore, the reactive components of the two sections are in all cases opposite in sign regardless of the amount or direction of the departure from the resonant frequency, and consequently tend to reduce each other in the complex impedance of the divider as a whole.

To illustrate: at the resonant frequency the crystal resonator presents an impedance of 4000 ohms and the circuit LCR 426,667 ohms, both purely resistive, making a total of 430,667 ohms. At :1706 cycles from resonance the crystal presents an impedance of 449,952 ohms, practically entirely reactive, while the impedance of the circuit LCR is equal to 301,700 ohms, consisting of a resistive component of 213,333 ohms and a reactive component of 213,333 ohms which is opposite in sign to that of the crystal reactance. The vector sum of these reactances and resistances is equal to 321,269 ohms.

As is well known in the art, the output voltage E is determined by the current flowing, as well as by the impedance presented by circuit LCR, and it is also well known that, again assuming a constant impressed voltage, the current flowing in the divider will be proportional to its total impedance. Therefore the current flowing in the divider at 11706 cycles from resonance will be times that flowing at resonance. Also at 11706 cycles from resonance the impedance of circuit LCR will be times its impedance at resonance. Therefore the output voltage E at 11706 cycles from resonance will be or 94.8% of the voltage E at exact resonance.

Thus with no additional resistance introduced into circuit LCR, my combination filter exhibits its minimum selectivity (maximum band width) Conversely, the introduction of additional resistance in series with L and C of Fig. 1A results in an increase of selectivity (decreased band width). While this efifect seems to be diametrically opposed to the general conception of the behavior of selective resonant circuits, the following example will illustrate how it is accomplished.

When R (22) in Fig. 1A is adjusted so as to introduce an additional resistance of 2000 ohms in series with L and C, the selectivity characteristic of my combination filter becomes materially difierent. From column IV of the impedance table it will be seen that the parallel impedance of circuit LCR at resonance has fallen to 5059 ohms and exhibits little change for frequency deviations up to 13000 cycles. Therefore, at resonance the total impedance of the divider will amount to 5059 plus 4000, or 9059 ohms, since both are pure resistance. At 1-1706 cycles from resonance the crystal impedance is 449,952 ohms, almost wholly reactive, and the impedance of circuit LCR is 5058 ohms, predominantly resistive. Their vector sum is accordingly 450,025 ohms, and the current flowing in the divider will be times the current at resonance. For the same frequency deviation the parallel impedance of the circuit LCR will remain substantially unchanged due to its high series resistance. Therefore, in this case the output voltage E at :1706 cycles from resonance will be equal to times the voltage E at resonance. This ratio amounts to 2% and indicates a tremendous increase in selectivity over that secured with no additional resistance in series with L and C.

From the above illustrations it becomes apparent that the addition of any intermediate amount of resistance between 0 and 2000 ohms in series with L and C of Fig. 1A will result in a corresponding intermediate degree of selectivity. Consequently by merely adjusting the setting of the variable resistor R (22), any degree of selectivity between the maximum and minimum possible with theparticular components used, may be secured. A further advantage lies in the fact that inasmuch as the Variations of impedance of both crystal resonator and circuit LCR are substantially equal for a given frequency departure, either above or below the resonant frequency, the resultant selectivity curve or frequency characteristic is symmetrical. It is advantageous in many types of practical applications of my combination filter, to maintain substantially constant output voltage. This may be secured by making the impedance of the secondary 3! of transformer 32 suificiently low so that its value is always considerably less than the 35 total impedance of the voltage divider regardless of the amount of resistance introduced into circuit LCR. If this is done, the output voltage E (at the resonant frequency) will vary but slightly for even large changes in selectivity. The choice of proper constants to aiiord other minima and maxima of selectivity, together with other desirable transmission characteristics of my combination filter, will be apparent to those skilled in the art.

Referring now to Fig. 3, I have here indicated one practical embodiment of my invention. The crystal filter 28 and the coil-condenser circuit 20, 2| are here shown connected in series, while resistance 22 is indicated as a tapped resistor, having a series of contact points and a movable arm 21', which latter is connected to junction point 27. The output of this filter is taken from points 26 and 21 and it can readily be seen that the combination filter is electrically equivalent to that shown in Fig. 1A. It is well known in the art that the crystal, together with its holder, possesses a certain unavoidable electrostatic capacity. It is also well known that the maximum filter action of such a device is only obtainable when means are employed to substantially counterbalance or neutralize such electrostatic capacity. One method of neutralizing this capacity is to employ a voltage directly opposite in phase to that of the signal voltage and equal thereto, impressed upon the terminal of the crystal filter which is connected to the output circuit.

In the circuit shown in Fig. 3 I have secured such a counter-phase voltage by means of condensers 4B and M which are of equal capacity and which are connected in series across the transformer secondary 3! at points 46 and 29 and consequently form a center-tap (point 42) across the secondary 3 I. The neutralizing condenser has one rotor 45 and two stators 41 and 48, and is of the opposed stator type, wherein the capacity between rotor 45 and stator- 4T plus the capacity between rotor 55 and stator 48 remains constant regardless of the angular position of rotor 45., The im portance of this'fature will appear later. Since the voltage appearing at point Q6 is equal to and opposite in phase to that appearing at point 29,

tion of the capacity between rotor t5 and stator d8 is exactly compensated by an equal and opposite change in capacity between rotor 45 and stator il, which is in series with condenser 44. If condenser M be made equal to condenser ll the tuning of circuit LCR is independent of the angular setting of the neutralizing condenser.

The importance of a constancy of overall capacity in this network can now be seen, since it is of course extremely undesirable that the resonant frequency to which circuit 20, 2! is tuned, should be alteredin any way. This feature of my invention thus allows me to vary the degree of crystal neutralization without efiecting the resonant frequency of the remaining portions of my combination filter.

In Fig. (i I have illustrated the filter circuit of Fig. 3 as actually co'nnectedat its input terminals to an electronic tube 46. I have also indicated the primary winding 33 of transformer 32 as tuned by a parallel connected condenser 41 so that it will present a suitably high impedance value with respect to the anode circuit of tube 46, out of which it is working. I have likewise indicated another electronic tube 48 to the control grid circuit of which are connected the output terminals 26 and 21 of my combination filter, so that the voltage E appears across the grid and cathode of tube ill. I have likewise indicated at 43 a ground connection made to one portion of my filter circuit. Such a ground connection I.

have found to be desirable, although not essential. It is to be understood that thevarious current sources for the electronic tubes maybe derived in any suitable fashion, as well known in the art.

Likewise it is to be understood that tube 48 may have its grid suitably biased by any appropriate means (not shown). While I have indicated resistance 22 as being of the continuously variable type, it is to be understood that this resistor may alternatively be of the tapped type shown in Fig. 3.

Referring now to Figs. 5 and 6, I have here shown transmission curves and the circuit from which they were derived. This circuit is substan tially equivalent to that in Fig. 4, but to which has been added a resistance 49, indicated as connected to a source of bias potential for the grid of tube 43, which is here designated as the 2nd I. F. tube of an intermediate frequency amplifier, of which tube 46 is the 1st I. F. tube. There is also shown a switch 50 connected in shunt with the crystal element 28. This switch may conveniently be .mechanically interconnected with the switch whose arm is connected to point 21, sov that when the arm of this latter tap switch is rotated to a predetermined position, it will cause the closing of switch 50 and the consequent short circuiting of crystal 28. Such a mechanical interconnection of two switches is well known in the radio art and since it does not form an essential feature of my invention I have thought it unnecessary to illustrate the mechanical details thereof. I have indicated resistance 22 as broken up into a series of smaller resistances R1, R2, R3, R4, by various taps which run to the switch points designated by reference numerals I, 2, 3, 4 and 5. The position of the switch indicated by the marking Off is the position in which it functions to actuate switch 50 and so to short circuit crystal 28. The selectivity curves of Fig. 5 bear indicia corresponding to the various points of the switch just described.

I have found that it is desirable to introduce a condenser 5| of relatively large capacity between load circuit connection point 21 and the ground. In this fashion a return circuit for the signal energy is provided through this condenser 5|, its ground connection and ground connection 43 which is connected to midpoint 42 of the input transformer secondary 3|. At the same time, a by-passing action is obtained by this condenser so that resistor 49 and the grid bias and other circuits connected thereto are removed from the direct path of the signal carrying current. It is to be understood that the use of such a condenser, as well as other details hereinafter to be described, does not, form an essential part of my invention but that I have found such an arrangement to contribute to the most desirable application of my invention.

Transformer 32 may be of any convenient type such as the permeability tuned type and its primary 33 may conveniently be adjusted by means of condenser 4'! so as to present a suitable load impedance for the output circuit of tube 46. If the transformer 32 is of the fixed inductance type, it is desirable that condenser 47 be made variable in order to allow such adjustment of the input impedance of the transformer. Secondary winding 3| is preferably made of comparatively low impedance so as to deliver a substantially constant voltage for the reasons previously given.

It will be noted that in this embodiment of my invention I have omitted the condenser 44 shown in Figs. 3 and 4. I have found that the ratio of ciently close balance of capacity will exist in this network without the use of this particular condenser. Crystal resonator 28 may be of any convenient type and is preferably cut and ground so that any spurious responses will be at least 10 kilocycles away from its resonant response. I have found it possible to provide a crystal of this type which will have no spurious response within 1 40 kc. Coil 20 may also be of the permeability tuned type and the resonant circuit through condenser 2I is likewise completed by means of a ground connection thereto.

A tap switch has been inserted at point 21 between the low-potential end of coil 20 and is grounded through condenser 5|. As already mentioned, this switch has six positions marked Off, I, 2, 3, 4 and 5. In the Off position (extreme counter-clockwise) the crystal holder is short-circuited by means of the two auxiliary contacts of switch 5!]. In switch position I this short-circuti is removed and the crystal feeds into the parallel tuned circuit L and C made up of elements 20 and H, which has previously been precisely tuned to the resonant frequency of the actual quartz crystal used. In this position of the switch, L and C present their maximum possible impedance as explained above in detail, and the crystal filter then provides the first step of increased selectivity over that obtainable from the tuned I. F. stages alone. In positions 2, 3, 4 and 5, successively greater amounts of resistance are introduced in series with L resulting in smaller values of Q and lower load impedance into which the crystal works. Each increase in series resistance causes a corresponding increase in selectivity until position 5 is reached. This position provides substantially the maximum selectivity of which the crystal is capable when used in the particular combination filter shown in this figure.

Accurate and elaborate tests of receivers constructed with the use of the combination filter circuit of my invention indicate that such a combination circuit actually will allow the degree of selectivity to be varied between the minimum and maximum limits previously described.

The phasing control works very symmetrically in the filter circuit of my invention. Its action is independent of the setting of the selectivity switch-another important advantage to the practical operator. When properly set, the crystal holder capacity is exactly neutralized, and the selectivity characteristics of the filter are truly symmetrical regardless of the position of the selectivity switch. When turned either side of this point of exact neutralization, there occurs a phenomenon familiar in the art. A rejection dip is introduced in the filter response curve, on one side or the other of resonance, depending on whether the control is turned above or below center scale. With the circuit of my invention these rejection settings are substantially independent of the position of the selectivity switch. For example, if the phasing control is set for rejection of a 1000 cycle beat note from an interfering transmitter, the degree of selectivity of the crystal can be altered at will with no substantial effect on the frequency of rejection, although the rejection dip in the response curve of the filter is deeper at the more selective settings of the selectivity switch.

Referring now especially to Fig. 5, the curves I, 2, 3, 4 and 5 are for the combination filter unit alone. When the tap switch is turned so as to connect to the points indicated by the numerals I, 2, 3, 4 and 5, the selectivity curves designated respectively by these same numbers in Fig. 5, will result from the characteristics of the filter presented when the resistances corresponding to these various positions are thus included in the circuit of coil 20 and condenser 2!. The dotted curve is that of a typical 2 stage tuned I. F. amplifier. The overall receiver selectivity would therefore be the sum of the dotted curve and any of the other curves, depending on the switch point used. The curves of Fig. 5 are not theoretically derived but represent the selectivity as actually measured. The testing of a receiver incorporating this combination filter circuit likewise indicated by means of oscillograms that the degree of symmetry was very great and therefore that interference due to asymmetrical characteristics is practically completely avoided by the use of my invention.

The curve indicated by the numeral I in Fig. 5

is not quite as wide as would be predicted from the theoretical analysis hereinbefore given. Part of the discrepancy is due to the fact that the selectivity characteristics of transformer 32 were not taken into account in making the calculations previously given, since constant output from this transformer was assumed. Additionally, it is probable that some or all of the constants assumed for the crystal and its load circuit were different in the filter actually measured, from those assumed in the discussion. However, it can readily be seen from inspection of the selectivity curves of Fig. 5 that the selectivity of a receiver employing this combination filter circuit of my invention may be adjusted by intermediate steps to any degree desired between that represented by the relatively low selectivity indicated by the curve marked I, and the extremely high degree of selectivity indicated by the curve marked 5. Of course it would be possible to choose the constants of the elements making up my combination filter circuit so that the minimum selectivity curve could be relatively much wider than that shown, if so desired. From the foregoing explanation of the principles involved, such an alteration could be readily made by one skilled in the art, in case that a receiver affording higher fidelity reception were desired.

I claim:

1. A filter combination including a piezo-electric crystal in series with a parallel tuned circuit, said latter circuit comprising inductance, capacity and resistance connected directly in series with one another, means for supplying energy of substantially predetermined frequencies to said series combination and means for withdrawing energy from said parallel tuned circuit portion only of said combination, said resistance being variable, and being so connected that the variation thereof will substantially alter the ratio of reactance to resistance in said tuned cir cuit while maintaining substantially constant the ratio of reactance to resistance in said piezo-electric crystal, whereby the effective band pass width of said filter combination can be varied at will.

2. The method of varying the effective selectivity of a frequency discriminating filter circuit including as one element a condenser and inductance coil connected in parallel with one another and including as another element a piezo-electric crystal in series with said first mentioned parallel connected combination, which comprises feeding energy to both said elements in series and varying the eifective resistance of said parallel connected condenser-coil combination while keeping the eifective resistance of said crystal constant and deriving the output of said circuit solely from said parallel connected element thereof.

3. A crystal filter circuit including a first variable capacity for neutralizing to any predetermined degree the effective capacity of the crystal and its ancillary apparatus and a second variable capacity for altering the capacity cross the output of said filter circuit, and including means for coupling both said variable capacities whereby both said capacities are simultaneously varied in opposite senses, so that the effective output capacity is substantially unaffected by neutralizing adjustments of said filter circuit.

4. A combination filter circuit including a resonant crystal circuit and first capacitive means for at least partially neutralizing the effective capacity of said crystal circuit, a second resonant circuit in series with said crystal circuit and second capacitive means for altering the resonant point of said second circuit, and

coupling means connecting both said capacitive means, so that the alteration of said first capacitive means will simultaneously alter in a cornpensatory direction said second capacitive means whereby the resonant point of said second circuit is maintained substantially constant when said first circuit is at least partially neutralized as to capacity.

5. A piezo-electric filter arrangement having input connections adapted to be supplied with high-frequency signalling energy and output connections adapted to supply output voltage to a translating device, comprising a piezo-electrio crystal and parallel tuned circuit connected in series between said input connections, and connections for impressing between said output connections the voltage developed across said parallel tuned circuit only, said parallel tuned circu1t including inductance and capacity in parallel with respect to current flowing through said crystal and being arranged to be fixedly anti-resonant to the series resonant frequency of said crystal, the impedance of said parallel tuned circuit being greater than the impedance of said crystal over a range of frequencies which is large compared to the range of frequencies over which the reactance of said crystal is smaller than its resistance, whereby signal frequencies throughout said first named larger range of frequencies are effectively transmitted through said filter.

6. A piezo-electric filter arrangement having input connections adapted to be supplied with high frequency signalling energy and output connections adapted to supply output voltage to a translating device, said filter comprising a piezoelectric crystal and a parallel tuned circuit connected in series between said input connections, connections for impressing between said output connections a voltage derived from said parallel tuned circuit, said parallel tuned circuit including inductance and capacity in parallel with respect to. current flowing through said crystal and being arranged to be fixedly anti-resonant to the series resonant frequency of said crystal, and resistive means for varying the impedance of said parallel tuned circuit while keeping its anti-resonant frequency'substantially constant whereby to vary the width of the band of frequencies eifece tively transmitted through said filter.

7. An electrical filter including inductance means coupled to asource of electrical energy, conductors coupled to at least a portion of said inductance means, a piezo-electric crystal having the first terminal connected to the first of said conductors, positive reactance means and nega-- tive reactance means both connected between the second of said conductors and the second terminal of said crystal, output leads connected respectively to the second of said conductors and to said second terminal of the crystal, and variable resistive means connected directly to at least one of said reactive means by a low impedance connection, whereby variation of said resistive means will alter the effective impedance of the circuit formed by said positive and negative reactance means while exerting substantially no effeet upon the effective impedance of said piezomeans for varying the Q of said second circuit including resistive means directly connected to said second circuit by a low impedance connection.

DONALD K. ORAM. 5 

